This invention relates to improvements in the production of perchloromethyl mercaptan. More particularly, it relates to the use of phosphates and/or phosphites as additives which serve to improve the yield of perchloromethyl mercaptan.
Perchloromethyl mercaptan, Cl.sub.3 CSCl, also known as trichloromethane sulfenyl chloride, has commercial importance as an intermediate in the manufacture of fungicides, bactericides, germicides, herbicides, soil fumigants and pharmaceuticals.
Perchloromethyl mercaptan was first described in a production scheme by Rathke in Annalen, Volume 167, at page 195 (1873). Rathke's method, which is still in use today, utilizes an iodine catalyst. The reaction scheme operates most efficiently at temperatures below about 40.degree. C., in accordance with the following equations: EQU CS.sub.2 + 3Cl.sub.2 .fwdarw. CCl.sub.3 SCl + SCl.sub.2 ( 1) EQU 2CS.sub.2 + 5Cl.sub.2 .fwdarw. 2CCl.sub.3 SCl + S.sub.2 Cl.sub.2 ( 2) EQU CS.sub.2 + 3Cl.sub.2 .fwdarw. CCl.sub.4 + S.sub.2 Cl.sub.2 ( 3)
In addition to sulfur dichloride, sulfur chloride (also known as sulfur monochloride) and carbon tetrachloride, the reaction can also form other compounds as unwanted byproducts. Although more volatile byproducts such as carbon tetrachloride and sulfur dichloride can be removed from the reaction mixture by distillation, it is extremely difficult to separate perchloromethyl mercaptan from sulfur chloride by this method. This is due to the fact that the boiling points of perchloromethyl mercaptan and sulfur chloride are very close to each other.
The prior art has proposed several methods for improving the basic Rathke method. For example, U.S. Pat. No. 3,544,625 to Masat, discloses a method for producing perchloromethyl mercaptan by chlorinating carbon disulfide in the presence of a solution of inorganic acids, such as hydrochloric acid. U.S. Pat. No. 3,673,246 to Meyer et al, discloses a continuous process for producing perchloromethyl mercaptan wherein carbon disulfide is reacted with chlorine on or in intimate contact with activated carbon at temperatures of about -5.degree. to +100.degree. C. U.S. Pat. No. 3,808,270 to Rupp et al, discloses a continuous process for producing perchloromethyl mercaptan by reacting carbon disulfide and chlorine in a reaction zone filled with granular active carbon completely immersed in the liquid reaction mixture while maintaining temperatures in the range of about 40.degree. to about 135.degree. C. U.S. Pat. No. 3,878,243 to Zupancic discloses a homogeneous catalyst system comprising a lead salt of a carboxylic acid which is soluble in carbon disulfide.
Notwithstanding the effectiveness of the above prior art patents as methods for producing perchloromethyl mercaptan (PMM), they do not deal with preventing the tendency of PMM to react with chlorine or sulfur dichloride to form carbon tetrachloride, sulfur, and sulfur monochloride. Mixtures of carbon disulfide, sulfur dichloride and perchloromethyl mercaptan also react in a similar fashion. The reactions which form carbon tetrachloride are believed to be accelerated by trace amounts of metals, such as iron, tin, and bronze, in the reaction mixture.
Small quantities of iron are generally present in the commercial carbon disulfide and chlorine used as reactants for PMM, at levels on the order of parts per million. The chlorine can be treated by passing it through a glass wool filter to remove most of the iron. However, the presence of iron at levels as low as one part per million can be deleterious and capable of effecting significant reductions in the yield of perchloromethyl mercaptan. It has, therefore, been an objective of industry to develop agents capable of ameliorating the effect of metallic impurities present in the reactants and/or catalyst, so that the formation of carbon tetrachloride, sulfur chloride, and other undesirable byproducts is suppressed.
Another problem in the production of perchloromethyl mercaptan occurs in the decomposition of sulfur dichloride to sulfur chloride and chlorine in the following manner: EQU 2SCl.sub.2 .revreaction. S.sub.2 Cl.sub.2 + Cl.sub.2 ( 4)
This reaction is undesirable due to the fact that the boiling points of perchloromethyl mercaptan and sulfur chloride are so close to each other that it is impractical to separate them by distillation. Thus, it has also been an objective of industry to develop agents for stabilizing sulfur dichloride to thereby prevent it from forming sulfur chloride and chlorine.
The present invention has achieved improvements in the production of perchloromethyl mercaptan via the use of phosphates and/or phosphites as additives which are believed to suppress the formation of the undesirable byproducts occurring in reactions (3) and (4).